Many devices have been proposed for safely grounding the housing of an electrical appliance, one of these devices is described in Canadian Patent No. 1,074,000 issued Mar. 18, 1980 to Helwig. This known apparatus comprises a cordless electrical appliance, an adaptor unit for the appliance, and a base unit. The base unit is provided with supply-voltage-connection contacts for connection to a supply voltage, and with two appliance-current-supply contacts. The base unit also includes an appliance-current-path connected between the supply-voltage-connection contacts and the appliance-current-supply contacts. In the base unit, a step down transformer with a secondary winding produces an auxiliary, low amplitude current, flowing through an auxiliary-current-path when a pair of auxiliary contacts are electrically interconnected, one of these auxiliary contacts being grounded. A relay is responsive to the auxiliary, low amplitude current in its control winding to close a relay switch connected in the appliance-current-path. The adaptor unit is provided with two appliance-current-supply contacts electrically connected to the internal, load circuit of the electrical appliance, and with a pair of auxiliary short-circuited contacts respectively engageable with the auxiliary contacts of the base unit for interconnecting the said pair of auxiliary contacts of the base unit. The appliance comprises an electrically conducting housing electrically connected to the short-circuited auxiliary contacts. When the adaptor unit is placed on the base unit, the appliance-current-supply contacts of the adaptor unit engages those of the base unit to supply with electric power the internal, load circuit of the appliance, the auxiliary contacts of the base unit are interconnected together to establish the auxiliary current, and the housing of the electrical appliance is grounded through the said grounded one of the auxiliary contacts of the base unit, and through the auxiliary contact of the adaptor unit connected to the latter grounded auxiliary contact of the base nit.
Although the above described prior art device permits to check appropriate connection between the auxiliary contacts of the base and adaptor unit, it cannot detect disconnection of the housing of the electrical appliance from the short-circuited auxiliary contacts, and therefore from the ground.